Sanders’ bid stuns no one, given his challenge to Hillary Clinton for the nomination in 2016 that turned out to be more formidable than most pundits predicted. But there have been surprises in what is likely to be biggest presidential field in American political history unfolds.

1. It’s a different Democratic Party

Bernie Sanders lost the nomination last time around, but he won the party.

"We were told that our ideas were radical and they were extreme," Sanders said in his announcement video, underscoring the point by making quotation marks in the air around the derisive adjectives. "Well, three years have come and gone, and as a result of millions of Americans standing up and fighting back, all of these policies and more are now supported by a majority of Americans."

Proposals he made that seemed controversial then — Medicare For All, which Hillary Clinton had dismissed as a pipe dream, and free tuition at public colleges — are now in the Democratic mainstream as the ideological center of the party has moved left. Indeed, progressives now looking for a champion have choices this time beyond Sanders, who describes himself as a Democratic socialist.

California Sen. Kamala Harris says she’s open to eliminating all private health insurance if a single-payer, government-run system could be created. Massachusetts Sen. Elizabeth Warren, decrying an economic system that she argues is “rigged” against working people, is calling for a populist “wealth tax” on the rich, a 2 percent charge on the holdings of those who have more than $50 million in assets.

That said, cracks are forming among the candidates over just how far left to go, especially as Trump paints the opposition party as radical socialists who would threaten traditional American values. “They’re slipping extremely far left,” the president warned at a Cabinet meeting last week. “We don’t want that to happen to our country.”

The ambitious “Green New Deal,” a nonbinding resolution unveiled by freshman New York Rep. Alexandria Ocasio-Cortez and veteran Massachusetts Sen. Edward Markey, could become a wedge between those who view it as an agenda and those who see it as an aspiration. It includes provisions to not only to address climate change but also to broaden the social safety net for Americans.

"I will tell you, I'm not a Democratic socialist," Harris told reporters in New Hampshire Monday. Pete Buttigieg, the mayor of South Bend, Ind., also sounded a note of caution, calling the Green New Deal "the right place to begin."

2. The candidates are more diverse

Pete Buttigieg, mayor of South Bend, Ind., answers questions from Iowans on Saturday, Feb. 9, 2018, in the living room of a home in Johnston. Buttigieg is exploring a run for president. (Photo: Kelsey Kremer/The Register)

Here’s another surprise: Of the 10 formal candidates so far, only two are straight white men. Put another way, eight of the 10 don’t fit the mold set by 43 of the previous 44 U.S. presidents. Four are women. Two are African-American. One is Hispanic. One is openly gay.

Buttigieg — who finesses the challenge of pronouncing his last name by introducing himself as “Mayor Pete” — exemplifies the redefinition of who could be seen as a credible contender. For one thing, he’s 37 years old; the youngest president, Teddy Roosevelt, was 42 when he took office. For another, Buttigieg is not a prominent senator or a governor but rather the mayor of a city of 102,000 Hoosiers. And he’s gay.

He has a standard response when interviewers ask him if America is ready to elect a gay president. “There’s only one way to find out,” he replies.

By the way, his husband, Chasten Buttigieg, has suggested pronouncing it “boot-edge-edge.” Or possibly “buddha-judge.” Or maybe “boot-a-judge.”

A young person holds a sign as Democratic Sen. Amy Klobuchar of Minnesota announces she is entering the race for president during a rally Sunday, Feb. 10, 2019, at Boom Island Park in Minneapolis. Jim Mone, AP

Snow falls as rally goers arrive at Boom Island Park for Democratic Sen. Amy Klobuchar's announcement of her decision in the race for president at a rally Sunday, Feb. 10, 2019, at Boom Island Park in Minneapolis. Jim Mone, AP

Volunteers Tim Schumann, left, and Chase Cushman move an "Amy for America" sign into place Sunday, Feb. 10, 2019, prior to Democratic Sen. Amy Klobuchar's announcement of her decision in the race for president at a rally in Minneapolis. Anthony Souffle, Star Tribune via AP

The combined North and Patrick Henry High School drum corps keeps warm in the parking lot Sunday, Feb. 10, 2019, before taking part in Sen. Amy Klobuchar's announcement to run for president from a snowy Boom Park, in Minneapolis. Glen Stubbe, Star Tribune via AP

Snow continues to fall as a worker clears snow off a runner and media members arrange equipment on a riser prior to Democratic Sen. Amy Klobuchar's announcement of her decision in the race for president at a rally Sunday, Feb. 10, 2019, at Boom Island Park in Minneapolis. Jim Mone, AP

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Former vice president Walter Mondale told Minnesota Sen. Amy Klobuchar he couldn’t quite believe she had scheduled her campaign announcement outdoors, in their native Minnesota, in February. While he’s endorsed her, the 91-year-old didn’t show up on Boom Island, but a blizzard did, blanketing Klobuchar’s uncovered head in a veil of snow as she declared her candidacy.

But what might have seemed like an advance planner's disaster turned out to be a political gold. Klobuchar came across as full of grit and good humor, despite the fierce weather — not bad qualities for a candidate to exhibit. Little known outside the North Star State, and dinged by stories that portray her as a tough boss, the photo of her battling the elements with a smile was a friendly first introduction to many voters.

That’s a good thing when authenticity is a calling card in politics today.

“By the end of her speech she looked like a Snowman(woman)!” President Trump, apparently unpersuaded, mocked in a tweet. She tweeted back: “And I wonder how your hair would fare in a blizzard?”

A counter-rally led by former U.S. Senate candidate Beto O'Rourke, holding his son Henry, joined the March for Truth with hundreds of people along with Border Network for Human Rights, Women's March, to protest against President Trump campaign rally in El Paso, Texas. NICK OZA, for USA TODAY

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Klobuchar didn’t mention the president's name when she formally announced her candidacy, though she may have had him in mind when she said Americans were “tired of the shutdowns and the showdowns, the gridlock and the grandstanding.” Warren didn’t mention his name, either, in the 4½ minute video she posted announcing formation of her exploratory committee.

Trump and just about everybody else expects him to be the Republican nominee, and the fevered opposition to him has fueled everything about the 2020 race.

But the lesson many take from the 2018 midterm elections and the 1998 midterms, waged during the impeachment of President Clinton, is that voters care more about what's going on in their own lives than the scandal engulfing a president. Most of the candidates have tried to emphasize what they're for, not just whom they're against.

While Sanders did target Trump by name, he also said that wasn't the only point. "Our campaign is not only about defeating Donald Trump, the most dangerous president in modern American history," he said. "Our campaign is about transforming our country."